Definition 3.1 ([1]}) 
Let \(P\)  be a right comodule algebra over a Hopf algebra
\(H\)  with bijective antipode, and let
\(B:=P^{\operatorname{co}H} := \lbrace  p \in P \, | \, \delta (p) = p \otimes 1 \rbrace .\) 
